Injection drug use is a major risk factor for acquisition of HIV infection in Russia where the prevalence of HCV among IV drug users can be as high as 93% and in the Perm Region the frequency of HCV coinfection among newly diagnosed HIV cases approaches 75%. Thus to understand better the pathogenesis of immune deficiency among injection drug users with HIV infection who are typically also HCV infected, it is important to have a better understanding of the determinants of immune perturbations in these populations. Attenuated CD4 T cell recovery during antiretroviral therapy (ART) administration leads to increased risk of mortality and non- AIDS-defining morbidities. As many as 30% of treated HIV infected patients experience poor CD4 restoration despite prolonged suppression of viral replication during ART and this is more likely among injection drug users, who are often HCV infected and who delay access to care. The mechanisms responsible for poor CD4 T cell recovery during ART have not been fully discerned but we have found in collaboration with Perm investigators that immune non responders (INR) have increased levels of immune activation, increased proportions of proliferating CD4+ T cells, increased expression of interferon-stimulated genes (ISGs) and perturbations in T regulatory cell numbers and via transcriptional analysis, impaired regulatory signatures and expression of genes involved in oxidative phosphorylation (preliminary data). We hypothesize that persistent exposure to the inflammatory type 1 IFN that is particularly increased in HCV coinfected injection drug users impairs Treg maturation and function resulting in dysregulated T cell homeostasis, characterized by non-productive T cell proliferation (rapid cell cycle entry followed by death), and ultimately, poor CD4 T cell reconstitution in INR. We propose that chronic exposure to inflammatory cytokines impairs mitochondrial mass and function in cycling cells leading to short lived proliferating cells. We will test this hypothesis by detailed transcriptional profiling of cycling and non-cycling T regulatory cells obtained from HIV infected and HIV/HCV coinfected patients receiving care in Perm using novel methods we have developed, and by detailed multiparameter flow cytometric characterization of Treg cells using a 16 color flow panel that we have optimized. We will also test the effects of interferons and other inflammatory mediators that we have found to be elevated in HIV/HCV coinfection on regulatory T cell maturation, phenotype and mitochondrial function to provide an in vitro system to explore the mechanisms of our model. The proposed studies will clarify poorly understood determinants of immune failure in HIV and HIV/HCV infection in a setting where treatment for HCV infection is not yet broadly available. Confirming the pathophysiologic mechanisms suggested by our preliminary data may identify targets for improving immune recovery in ART-treated HIV and HIV/HCV coinfection .